Distribution and abundance of juvenile starfish in the central Great Barrier Reef

The densities of juvenile Acanthaster planci (Crown-of thorns starfish) were sampled at 16 inner mid-shelf reefs of the central Great Barrier Reef during trips in July, September and November 1986 and were resurveyed in November/December 1987. The reefs sampled were: Lodestone, Keeper, Little Broadhurst, Bowden, Mid, Stanley (3 independent surveys), Old, Charity, Hope, Showers, Bait, Ross, Square, Robertson, Credlin and Bushy-Redbill.

A destructive sampling technique was chosen due to the small size and cryptic behaviour of the starfish during their first 10-12 months of benthic development. On each reef two replicate 10x1m transects were sampled at each of two arbitrarily chosen sites located within three depth zones (reef base, reef crest and reef flat) and two exposure categories (windward and leeward). Surface features of each transect were mapped with a resolution of 15cm. Loose and detachable pieces of substrate were removed, broken into small pieces and searched underwater for the presence of stafish. While the study was primarily focused on Acanthaster planci, all starfish species were recorded. The position of each starfish encountered was recorded on the substratum map and the specimen retained for subsequent identification and measurement. Sampling of a transect ceased either when it reached bed rock or a depth of 30cm.

In order to determine the precision of the sampling method, the reef with the highest densities observed on the first cruise in 1986 (Stanley Reef) was resurveyed twice more during subsequent cruises that year.

In both 1986 and 1987, transects at one leeward site on each of four reefs were sampled nocturnally. Twelve 12 transects were marked out and 6 were sampled in the usual way during the day and 6 were searched without destruction after 2100 hours.